The present invention relates to a phase demodulator for demodulating digital-signal-modulated carrier waves by phase detection.
Digital-signal-modulated carrier waves such as those provided by-the phase shift keying (PSK), quadrature amplitude modulation (QAM), or the like are generally demodulated by orthogonal phase detectors. The orthogonal phase detectors are individually supplied with reference carrier waves which are orthogonal to each other, that is, one is supplied directly with a reference carrier itself and the other with the reference carrier through a 90-degrees phase shifter. When such phase detectors are supplied with modulated carrier waves of the same phase, they multiply the modulated carrier waves by the reference waves coupled thereto and thereby produce at their output terminals demodulated signals which are orthogonal to each other.
In a phase demodulator of the type described, deterioration in bit error rate has to be eliminated in order that the reference carrier waves fed to the two phase detectors may be held orthogonal to each other, as will be described later in detail. A 90-degree phase shifter in practical use cannot meet such a demand because the phase is susceptive to variations in ambient temperature, power source voltage, and to a long-term variation in circuit constant.